Adding a node to a running replicaset on an existing shard - mongodb

Question

MongoDB shard needs to know about the members of a replicaset. Is the member list discovery dynamic? I mean if we add a node to an existing replicaset which is already configured as a shard on the config servers, does the shard automatically update or do we have do manually update the shard configuration with any new member added to the replica?

Answer 1

In older versions of Mongo, prior to 2.0.3 all of the replica set members needed to be specified when adding a shard. Thus it seems fair to conclude that when a shard is added and it only needs to know one of the members of the replica set, then all activity between replica set members is delegated to the replica set.
Probably the optimal way to be sure is fire up a test scenario on your own machine. But there is nothing to suggest there is any additional configuration to sharding that should be required.
And as a bit of an update as I had nothing to do over having lunch :) I just spun up the a load of instances as mapped out in the listed tutorial:
http://docs.mongodb.org/manual/tutorial/add-shards-to-shard-cluster/
A few differences being use of sh.addShard in the first member of replica set added only, rather than the all members syntax shown in the docs.
Once the shards were up. I just added two more replica set nodes to the firstset.
http://docs.mongodb.org/manual/tutorial/expand-replica-set/
Without anything else let's see the status from mongos
mongos> db.printShardingStatus()
--- Sharding Status ---
sharding version: {
"_id" : 1,
"version" : 4,
"minCompatibleVersion" : 4,
"currentVersion" : 5,
"clusterId" : ObjectId("52f2f77a538f784f4413e6b9")
}
shards:
{ "_id" : "firstset",
"host" :"firstset/localhost:10001,localhost:10002,localhost:10003,localhost:10004,localhost:10005" }
{ "_id" : "secondset", "host" : "secondset/localhost:20001,localhost:20002,localhost:20003" }
databases:
{ "_id" : "admin", "partitioned" : false, "primary" : "config" }
{ "_id" : "test", "partitioned" : true, "primary" : "firstset" }
test.test_collection
shard key: { "number" : 1 }
chunks:
secondset 23
firstset 191
So the shard is still moving chunks and the new nodes just finsihed initializing as I was typing.
And that's all there is to adding additional nodes to a replica set on a shard. Most of this was done during a 1 million document insert.